No abstract

It has been known since the 1960s that the layers of Na and K atoms, which occur between 80 and 105 km in the Earth's atmosphere as a result of meteoric ablation, exhibit completely different seasonal behavior. In the extratropics Na varies annually, with a pronounced wintertime maximum and summertime minimum. However, K varies semiannually with a small summertime maximum and minima at the equinoxes. This contrasting behavior has never been satisfactorily explained. Here we use a combination of electronic structure and chemical kinetic rate theory to determine two key differences in the chemistries of K and Na. First, the neutralization of K + ions is only favored at low temperatures during summer. Second, cycling between K and its major neutral reservoir KHCO 3 is essentially temperature independent. A whole atmosphere model incorporating this new chemistry, together with a meteor input function, now correctly predicts the seasonal behavior of the K layer.

The National Aeronautics and Space Administration (NASA) Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) Sounding of the Atmosphere using Broadband Radiometry (SABER) instrument performs near‐global measurements of the vertical kinetic temperature (Tk) profiles and volume mixing ratios of various trace species (including O3, CO2, and H2O), with data available from 2002 to present. In this work, the first comparative study of the latest publically available SABER version 2.0 operational retrieval is reported in order to assess the performance of satellite Tk profiles relative to high‐resolution ground‐based lidar profiles. Collocated multiyear seasonal average Tk profiles were compared at nine different locations, representing a variety of different latitudes. In general, the SABER v2.0 and lidar mean seasonal Tk profiles agree well, with the smallest absolute values of ΔTk (z) (SABER minus lidar) found between 85 and 95 km, where the respective SABER and lidar uncertainties were smallest. At altitudes ≥100 km, the SABER Tk (z) typically exhibited warmer temperatures relative to the lidar Tk (z) profiles, whereas for altitudes ≤85 km, SABER Tk (z) was cooler. Relative to lidar, SABER tends to exhibit a warm bias during high‐latitude summertime, with the reasons for this currently still unclear. Overall, SABER was able to reproduce the general latitude‐ and season‐specific variations in the lidar Tk profiles and shown to be statistically similar for most seasons, at most locations, for the majority of altitudes, and with no overall bias.

Precipitation is an important component of the climate system, and the accurate representation of the diurnal rainfall cycle is a key test of model performance. Although the modeling of precipitation in the cooler midlatitudes has improved, in the tropics substantial errors still occur. Precipitation from the operational ECMWF forecast model is compared with satellite-derived products from the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) and TRMM Precipitation Radar (PR) to assess the mean annual and seasonal diurnal rainfall cycles. The analysis encompasses the global tropics and subtropics (40°N–40°S) over a 7-yr period from 2004 to 2011. The primary aim of the paper is to evaluate the ability of an operational numerical model and satellite products to retrieve subdaily rainfall. It was found that during the first half of the analysis period the ECMWF model overestimated precipitation by up to 15% in the tropics, although after the implementation of a new convective parameterization in November 2007 this bias fell to about 4%. The ECMWF model poorly represented the diurnal cycle, simulating rainfall too early compared to the TMPA and TRMM PR products; the model simulation of precipitation was particularly poor over Indonesia. In addition, the model did not appear to simulate mountain-slope breezes well or adequately capture many of the characteristics of mesoscale convective systems. The work highlights areas for further study to improve the representation of subgrid-scale processes in parameterization schemes and improvements in model resolution. In particular, the proper representation of subdaily precipitation in models is critical for hydrological modeling and flow forecasting.